Senior Unsecured Fixed Rate Notes - Condensed Consolidating Statement of Cash Flows (Detail) - USD ($) $ in Millions	9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019
Condensed Financial Statements, Captions [Line Items]
Net cash flows from operating activities	$ 101.4	$ 195.7
Cash flows from investing activities:
Proceeds from sales of investments	1,091.2	1,180.3
Maturities and mandatory calls of fixed maturity investments	410.3	292.2
Purchases of investments	(1,752.1)	(1,639.5)
Change in short-term investments and foreign regulatory deposits	297.3	2.3
Settlements of foreign currency exchange forward contracts	9.1	(1.2)
Capital contribution to subsidiaries	0.0
Sale of Trident's assets	38.0	0.0
Purchases of fixed assets and other, net	(13.1)	6.7
Cash provided by (used in) investing activities	80.7	(159.2)
Cash flows from financing activities:
Payment of long-term debt	(125.0)	(0.6)
Issuance of preferred shares, net of issuance costs	144.0	0.0
Capital contribution from parent	0.0
Payment on the intercompany note		0.0
Activity under stock incentive plans	1.4	1.4
Payment of cash dividends to preferred shareholders	(2.0)	0.0
Payment of cash dividends to common shareholders	(32.2)	(32.5)
Cash provided by (used in) financing activities	(13.8)	(31.7)
Effect of exchange rate changes on cash	(5.7)	1.1
Change in cash	162.6	5.9
Cash, beginning of year	137.8	139.2
Cash, end of period	300.4	145.1
Argo Group International Holdings, Ltd. (Parent Guarantor)
Condensed Financial Statements, Captions [Line Items]
Net cash flows from operating activities	35.6	29.2
Cash flows from investing activities:
Proceeds from sales of investments	0.0	0.0
Maturities and mandatory calls of fixed maturity investments	0.0	0.0
Purchases of investments	0.0	0.0
Change in short-term investments and foreign regulatory deposits	(0.6)	3.5
Settlements of foreign currency exchange forward contracts	0.1	(0.1)
Capital contribution to subsidiaries	(145.3)
Sale of Trident's assets	0.0
Purchases of fixed assets and other, net	0.0	0.0
Cash provided by (used in) investing activities	(145.8)	3.4
Cash flows from financing activities:
Payment of long-term debt	0.0	0.0
Issuance of preferred shares, net of issuance costs	144.0
Capital contribution from parent	0.0
Payment on the intercompany note		0.0
Activity under stock incentive plans	1.4	1.4
Payment of cash dividends to preferred shareholders	(2.0)
Payment of cash dividends to common shareholders	(32.2)	(32.5)
Cash provided by (used in) financing activities	111.2	(31.1)
Effect of exchange rate changes on cash	0.0	0.0
Change in cash	1.0	1.5
Cash, beginning of year	1.9	1.7
Cash, end of period	2.9	3.2
Argo Group US, Inc. and Subsidiaries (Subsidiary Issuer)
Condensed Financial Statements, Captions [Line Items]
Net cash flows from operating activities	23.5	124.6
Cash flows from investing activities:
Proceeds from sales of investments	581.6	744.2
Maturities and mandatory calls of fixed maturity investments	296.3	219.2
Purchases of investments	(1,294.2)	(1,091.2)
Change in short-term investments and foreign regulatory deposits	449.9	34.7
Settlements of foreign currency exchange forward contracts	(0.1)	1.4
Capital contribution to subsidiaries	0.0
Sale of Trident's assets	38.0
Purchases of fixed assets and other, net	7.0	(1.8)
Cash provided by (used in) investing activities	78.5	(93.5)
Cash flows from financing activities:
Payment of long-term debt	0.0	(0.6)
Issuance of preferred shares, net of issuance costs	0.0
Capital contribution from parent	0.0
Payment on the intercompany note		(19.1)
Activity under stock incentive plans	0.0	0.0
Payment of cash dividends to preferred shareholders	0.0
Payment of cash dividends to common shareholders	0.0	0.0
Cash provided by (used in) financing activities	0.0	(19.7)
Effect of exchange rate changes on cash	0.0	0.0
Change in cash	102.0	11.4
Cash, beginning of year	31.6	31.7
Cash, end of period	133.6	43.1
Other Subsidiaries and Eliminations
Condensed Financial Statements, Captions [Line Items]
Net cash flows from operating activities	42.3	41.9
Cash flows from investing activities:
Proceeds from sales of investments	509.6	436.1
Maturities and mandatory calls of fixed maturity investments	114.0	73.0
Purchases of investments	(457.9)	(548.3)
Change in short-term investments and foreign regulatory deposits	(152.0)	(35.9)
Settlements of foreign currency exchange forward contracts	9.1	(2.5)
Capital contribution to subsidiaries	0.0
Sale of Trident's assets	0.0
Purchases of fixed assets and other, net	(20.1)	8.5
Cash provided by (used in) investing activities	2.7	(69.1)
Cash flows from financing activities:
Payment of long-term debt	(125.0)	0.0
Issuance of preferred shares, net of issuance costs	0.0
Capital contribution from parent	145.3
Payment on the intercompany note		19.1
Activity under stock incentive plans	0.0	0.0
Payment of cash dividends to preferred shareholders	0.0
Payment of cash dividends to common shareholders	0.0	0.0
Cash provided by (used in) financing activities	20.3	19.1
Effect of exchange rate changes on cash	(5.7)	1.1
Change in cash	59.6	(7.0)
Cash, beginning of year	104.3	105.8
Cash, end of period	163.9	98.8
Consolidating Adjustments
Condensed Financial Statements, Captions [Line Items]
Net cash flows from operating activities	0.0	0.0
Cash flows from investing activities:
Proceeds from sales of investments	0.0	0.0
Maturities and mandatory calls of fixed maturity investments	0.0	0.0
Purchases of investments	0.0	0.0
Change in short-term investments and foreign regulatory deposits	0.0	0.0
Settlements of foreign currency exchange forward contracts	0.0	0.0
Capital contribution to subsidiaries	145.3
Sale of Trident's assets	0.0
Purchases of fixed assets and other, net	0.0	0.0
Cash provided by (used in) investing activities	145.3	0.0
Cash flows from financing activities:
Payment of long-term debt	0.0	0.0
Issuance of preferred shares, net of issuance costs	0.0
Capital contribution from parent	(145.3)
Payment on the intercompany note		0.0
Activity under stock incentive plans	0.0	0.0
Payment of cash dividends to preferred shareholders	0.0
Payment of cash dividends to common shareholders	0.0	0.0
Cash provided by (used in) financing activities	(145.3)	0.0
Effect of exchange rate changes on cash	0.0	0.0
Change in cash	0.0	0.0
Cash, beginning of year	0.0	0.0
Cash, end of period	$ 0.0	$ 0.0